Line coding is a process by which a communication protocol arranges symbols that represent binary data in a particular pattern for transmission. Conventional line coding used in fiber optic communications includes non-return-to-zero (NRZ), return-to-zero (RZ), and biphase, or Manchester. The binary bit stream derived from these line codes can be directly modulated onto wavelengths of light generated by the resonating frequency of a laser. Traditionally direct binary modulation based transmission offers an advantage with regard to the acceptable signal-to-noise ratio (SNR) at the optical receiver, which is one of the reasons direct binary modulation methods are used in the Datacom Ethernet/IP, Storage Fiber-Channel/FC and Telecom SONET/SDH markets for transmission across nonmultiplexed unidirectional fiber links.
The performance of a fiber optic network can be measured by the maximum data throughput rate (or information carrying capacity) and the maximum distance between source and destination achievable (or reach). For Passive Optical Networks (PONs) in particular, additional measures of performance are the maximum number of Optical Networking Units (ONUs) and/or Optical Networking Terminals (ONTs) possible on a network and the minimum and maximum distance between the Optical Line Terminator (OLT) and an ONU/ONT. These performance metrics are constrained by, among other things, amplitude degradation and temporal distortions as a result of light traveling through an optical fiber.
Amplitude degradation is substantially a function of length or distance between two end points of an optical fiber. Temporal distortion mechanisms include intramodal (chromatic) dispersion and intermodal (modal) dispersion. Intramodal dispersion is the dominant temporal dispersion on Single-mode fiber (SMF), while intermodal dispersion is dominant on Multi-mode fiber (MMF). Both types of temporal distortions are measured as functions of frequency or rate of transmission (also referred as line rate of a communication protocol) over distance in MHz·km. Temporal distortions are greater, hence a constraint on network performance, with increasing frequency transmission.